British actors "really freak out" when given the freedom to change their lines, top US writer-producers have said.

American actors do not always stick to the script the way their UK counterparts do, Lee Daniels, executive producer of Fox's hit drama Empire, told a Hollywood Radio and Television Society panel discussion in Beverly Hills.

During production of Daniels' 2013 film The Butler, veteran British actor Alan Rickman, playing President Ronald Reagan, "was married to the word", he said.

"I said, 'You don't have to say the word. And he was like, 'No, this is the word'," Daniels said.

He said he was willing to acknowledge that his scripts had room for improvement and that actors could add nuance to the characters they knew so well.

Taraji P Henson, who plays matriarch Cookie Lyon in Empire, will "add a line or word that makes it sparkle", Daniels said.

The Affair producer Sarah Treem agreed with Daniels' assessment. The playwright, whose other TV credits include House Of Cards, produces Showtime's drama series with a cast that includes American actors Maura Tierney and Joshua Jackson and Britons Dominic West and Ruth Wilson.

"British actors really like the text. They practise the text, and they're perfect on the text," she said. In contrast, Tierney was a "genius" at improvising, she said - prompting the script supervisor to ask if the actress should be told to refrain.

"No, let the woman speak," was Treem's reply. "She'll come up with something that is honestly more instinctive and more natural than what's on the page."

She noted another national difference, involving America and France. Treem said "hate mail" was posted online after she gave a US interview in which she questioned whether monogamy could hold in a long-term relationship.

"I was on the phone talking with somebody from France about the show and they had this totally different perspective. It's great talking to the French," she said.

In the event that included producers from CBS' The Good Wife, FX's Fargo and Amazon Studios' Transparent, the panellists were asked about a dream writing project they have yet to pursue.

"I haven't been afraid since Rosemary's Baby or The Exorcist," Daniels said. "And I don't think there's been an African-American horror movie that's really got me to the bones. I think I'd like to tackle that."